I had this happen last year and acted too slowly for one baby. I lost a triple gene Mojave female but I was able to save her sibling Mojave YB. I simply sterilized my hands and undid the tight part where it was twisted on the umbilicus. I massaged the yolk up the umbilicus and into the babies belly carefully. I then used sterilized scissors to cut the umbilical about an 1.25" below the belly and tied it off with new waxed dental floss. You do not want to tie it up too far or it can be sucked up into the belly and present a whole different problem. I kept her in the incubator in an ICU cup (small packed with moist paper towels) until the umbilical healed and fell off. Today she's one of my best eating 2012 hatchlings and a fine example of a Mojave YB. Because I was able to get the yolk into her stomach I did not have to assist feed. She eagerly took food 10 days after the "surgery".
Regards,
B